Washington, Mar 17 (IANS): Osama bin Laden was focused on a plan to assassinate President Barack Obama, the Washington Post said Friday, citing documents seized by the US military team that killed the Al Qaeda leader in May 2011.

Bin Laden's idea called for shooting down the aircraft of the president and of Gen. David Petraeus, who from July 2010 to July 2011 was commander of US forces in Afghanistan.

The Al Qaeda chief explained the focus on Obama and Petraeus in a message to his senior lieutenant.

"Obama is the head of infidelity and killing him automatically will make (Vice President Joe) Biden take over the presidency ... Biden is totally unprepared for that post, which will lead the US into a crisis. As for Petraeus, he is the man of the hour ... and killing him would alter the war's path" in Afghanistan, Bin Laden wrote.

US officials told the Post that the plan to kill Obama and Petraeus was never a serious threat, as Al Qaeda lacks the capacity to carry out such attacks.

Bin Laden told deputy Atiyah Abd al-Rahman that Pakistani terrorist Ilyas Kashmiri should take charge of the Obama-Petraeus assassination project.

Kashmiri was killed in a US drone strike a month after Osama's death.

The Al Qaeda leader urged his followers to concentrate on organising attacks in the US instead of spilling "Muslim blood" in operations in the Middle East and South Asia.

The documents taken from Osama's compound in Pakistan will soon be released to the public in both the original Arabic and English translation, the Washington Post said.